Crucial Role of the Residue R280 at the F′–G′ Loop of the Human Granulocyte/Macrophage Colony-stimulating Factor Receptor α Chain for Ligand Recognition

The receptor for granulocyte/macrophage colony-stimulating factor (GM-CSF) is composed of two chains, α and βc. Both chains belong to the superfamily of cytokine receptors characterized by a common structural feature, i.e., the presence of at least two fibronectin-like folds in the extracellular domain, which was first identified in the growth hormone receptor. The GM-CSF receptor (GMR)-α chain confers low affinity binding only (5–10 nM), whereas the other chain, βc, does not bind GM-CSF by itself but confers high affinity binding when associated with GMR-α (25–100 pM). The present study was designed to define the assembly of the GMR complex at the molecular level through site-directed mutagenesis guided by homology modeling with the growth hormone receptor complex. In our three-dimensional model, R280 of GMR-α, located in the F′–G′ loop and close to the WSSWS motif, is in the vicinity of the ligand Asp112, suggesting the possibility of electrostatic interaction between these two residues. Through site directed mutagenesis, we provide several lines of evidence indicating the importance of electrostatic interaction in ligand–receptor recognition. First, mutagenesis of GMR-αR280 stri